The invention relates to medical devices, and in particular, to power sources for portable defibrillators.
Cardiac arrest and ventricular fibrillation are life-threatening medical conditions that may be treated with external defibrillation. External defibrillation includes applying electrodes to the patient""s chest and delivering an electric shock to the patient to depolarize the patient""s heart and restore normal sinus rhythm. The chances that a patient""s heart can be successfully defibrillated increase significantly if a defibrillation shock is applied quickly. In many cases, it is more expedient to bring a defibrillator to the patient than to bring the patient to a defibrillator.
Many external defibrillators are portable. Portable external defibrillators may be used in hospitals and outside hospital settings as well. Paramedics, emergency medical technicians and police officers, for example, may carry portable external defibrillators in their vehicles. In addition, automated external defibrillators (AED""s) may be available in public venues such as airports, health clubs and auditoriums. Portable external defibrillators are compact and lightweight.
Before an external defibrillator is used to administer a shock, the energy to be delivered to the patient must be stored in an energy storage device, such as a capacitor. Many defibrillators use a charging circuit to transfer energy from a power source, such as an electrical outlet or a battery, to the energy storage device. When a switch is closed, the energy storage device delivers at least a part of the stored energy from electrode to electrode through the patient""s chest. Delivery of energy from the energy storage device is completed in a few milliseconds.
Following administration of one shock, it may be necessary to administer another. The charging circuit draws energy from the power source and transfers the energy to the energy storage device. When the energy storage device is sufficiently charged, another shock may be administered. In order to charge the energy storage device quickly, the power source may be called upon to supply approximately fifty to one hundred fifty watts of power. The power supplied is a function of the voltage of the power source and the current supplied by the power source. To supply fifty to one hundred fifty watts, a ten-volt power source, for example, may be called upon to supply between five to fifteen amperes of current.
Some conventional external defibrillators are powered by a connection to an electrical outlet. These xe2x80x9cline-poweredxe2x80x9d defibrillators carry risks of injury to the patient, to bystanders and to persons operating the defibrillator. In particular, a line-powered defibrillator carries a risk of sending line power to the patient and a risk of sending high voltage to the power cord during delivery of the defibrillation shock. Accordingly, a line-powered defibrillator usually includes electrical isolation circuitry to prevent the line power from reaching the patient and to prevent the defibrillation energy from passing through the power cord. Some isolation circuitry, such as a transformer, is large and heavy, adversely affecting the portability of the external defibrillator. In addition, a line-powered defibrillator cannot draw energy from an electrical outlet when the patient is far from an electrical outlet.
Other external defibrillators use one or more batteries as a power source. A battery-powered defibrillator does not require a power cord and therefore does not require the bulky isolation circuitry needed for a power cord. In conventional external defibrillators, batteries may be permanently mounted or replaceable.
When a defibrillator includes batteries that are permanently mounted in the device, the batteries usually are rechargeable. Many rechargeable batteries, such as nickel-cadmium batteries, sealed lead acid batteries or nickel-metal-hydride batteries, require a special recharging apparatus. The addition of recharging apparatus to the defibrillator adds to the bulk, weight and cost of the device. In addition, the recharging apparatus may be line-powered, which in turn necessitates electrical isolation circuitry for safety.
A further drawback to rechargeable batteries is short shelf life. Nickel-metal-hydride batteries, for example, discharge within a few months, even when no load is applied. Some rechargeable batteries, such as nickel-cadmium batteries, need to undergo conditioning cycles periodically to deliver optimum performance.
External defibrillators may include capacitors to smooth the power supplied by the rechargeable batteries. Although capacitors can be used as a rechargeable source of dc power, banks of conventional capacitors are incapable of supplanting the rechargeable batteries as a power source. Banks of conventional capacitors store too little energy, or are too bulky for a portable external defibrillator, or both.
As an alternative to permanent rechargeable batteries, a defibrillator may use replaceable batteries as a power source. Many replaceable batteries are not capable of delivering the energy demanded by the charging circuit. Replaceable batteries that can deliver the energy typically require heavy duty, low impedance connectors to carry the high currents safely, and the bulky connectors add to the weight and cost of the device. Heavy duty, low impedance conductors also increase the risk of inadvertent shock to an operator, because of the comparatively large size of the conductors.
In addition, some replaceable high capacity batteries pose additional hazards. When lithium sulfur dioxide batteries fail, for example, the batteries vent noxious gases, and when lithium manganese batteries fail, the batteries vent flammable electrolytes.
In general, the invention uses a power source that includes a first power pack and a second power pack for use in supplying energy to the energy storage device of a defibrillator. The first power pack is replaceable and the second power pack is rechargeable. The replaceable first power pack is used to charge the rechargeable second power pack. The replaceable first power pack may also be used to maintain the charge of the second power pack. When a charging circuit draws energy from the power source to charge an energy storage device associated with the defibrillator, the charging circuit draws the energy principally from the rechargeable second power pack.
The replaceable first power pack and rechargeable second power pack cooperate to provide the energy needed by the charging circuit to charge the energy storage device. In one exemplary embodiment, the replaceable first power pack comprises batteries, rendering unnecessary the electrical isolation circuitry that would be present in a line-powered defibrillator.
Furthermore, the first power pack recharges the second power pack without the need for a special charging device. In a typical implementation, the first power pack charges the second power pack via a parallel electrical connection. Because no special charging device is needed, the power source is smaller and lighter, and the defibrillator is more portable.
The first power pack may include one or more lithium thionyl chloride (Li/SOCl2) batteries. In one exemplary embodiment, the first power pack includes three lithium thionyl chloride batteries.
The second power pack may include one or more lithium ion batteries. In one exemplary embodiment, the second power pack includes six lithium ion batteries, with three pairs of lithium ion batteries coupled in parallel.
Lithium thionyl chloride batteries and lithium ion batteries work well together. The batteries can be selected to have matched working voltages and can operate over a wide temperature range. Both kinds of batteries are small and light, and help make the defibrillator small, light and portable. Working together, the batteries have a very long shelf life and can supply energy even if the defibrillator has been idle for an extended period. In addition, neither battery vents harmful electrolytes in case of failure.
A replaceable first power pack such as a lithium thionyl chloride battery may have a low power output and may be unable supply high amperages. In that case, heavy duty, low impedance connectors are not needed to couple the first power pack to the second power pack. Because of its low power output, the first power pack cannot supply the current demanded by the charging circuit. The first power pack can, however, charge a second power pack, such as a lithium ion battery, at a comparatively slow rate.
The second power pack may include one or more capacitors. Like rechargeable batteries, capacitors can be charged, used as a direct-current power source, and recharged. In one exemplary embodiment, the second power pack includes ultracapacitors, which are compact capacitors with high energy density. The ultracapacitors may be grouped into one or more banks of ultracapacitors, with the banks of ultracapacitors coupled in parallel with the replaceable first power pack. The second power pack may include both batteries and capacitors.
The second power pack generally has a much higher power output than the first power pack. The second power pack can therefore supply the current demanded by the charging circuit to charge the energy storage device quickly. The high-current connectors that conduct that current are shielded from the operator inside the case of the defibrillator.
In one embodiment, the invention is directed to a defibrillator that includes a replaceable first battery coupled to a rechargeable second battery. The first battery may comprise a lithium thionyl chloride battery and the second battery may comprise a lithium ion battery. The second battery may be permanently mounted inside the defibrillator. In ordinary use, the replaceable first battery and the rechargeable second battery may be coupled in parallel.
In another embodiment, the invention is directed to a device comprising a power source that supplies energy to an energy storage device in a defibrillator. The power source comprises a replaceable first power pack coupled to a rechargeable second power pack. Furthermore, the replaceable first power pack comprises at least one lithium thionyl chloride battery and the rechargeable second power pack comprises at least one lithium ion battery.
In an additional embodiment, the invention is directed to a defibrillator that includes a replaceable first power pack coupled to a rechargeable second power pack. The second power pack includes a rechargeable battery, a capacitor, or both, and stores at least 100 joules. The second power pack may include one or more ultracapacitors.
In a further embodiment, the invention is directed to a method comprising charging a second power pack with a replaceable first power pack in parallel with the second power pack, charging an energy storage device with energy from the second power pack and delivering energy from the energy storage device to a defibrillation electrode. Charging the second power pack may include, for example, charging a rechargeable battery or an ultracapacitor in the second power pack.
In an added embodiment, the invention is directed to a method comprising uncoupling a first power pack from a rechargeable second power pack in a defibrillator and coupling a fresh first power pack to the rechargeable second power pack. This method may also include replacing the electrodes on the defibrillator.
As noted above, the replaceable first power pack and rechargeable second power pack eliminate the need for some electrical isolation circuitry, special charging apparatus and heavy-duty connectors. Each of these advantages contributes to making the defibrillator portable and safe. In addition, lithium thionyl chloride batteries and lithium ion batteries are small, light and vent no harmful substances, further contributing to safety and portability. The batteries also have a long shelf life, can have matched working voltages and operate well over a wide temperature range.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.